I realize that when nameservers are changed it may take up to 48 hours for the change to "propogate". Does this mean that some (or possibly many) people may not be able to access a site that has had the nameservers changed? I always thought this was seamless.
It should be seamless. Once the nameservers are updated visitors will be sent to the new location instead of the old. While they are not updated, people still go to the old. There shouldn't be any down time.